Ta2O5 Films Research Articles

Antibacterial and biological characteristics of tantalum oxide coated titanium pretreated by plasma electrolytic oxidation

In this study, amorphous tantalum oxide coatings were deposited onto the Ti substrate with and without plasma electrolytic oxidation (PEO)-pretreatment by using a high-powered impulse magnetron sputtering system. The antibacterial tests were conducted using Syto9 fluorescent dyes to study the antibacterial performance of Gram-positive (Staphylococcus aureus) and Gram-negative (Actinobacillus actinomycetemcomitans) bacteria. In vitro studies, the cell cytotoxicity and cell viability were evaluated with soft-tissue derived human skin fibroblasts and hard-tissue derived human osteosarcoma cells. The results of the antibacterial experiments revealed that both the Ta2O5 and PEO-pretreated Ta2O5 samples had superior antibacterial properties compared to the uncoated Ti samples. Cell viability analyses showed that the Ta2O5 films with and without PEO pretreatment exhibited better cell growth compared to the Ti specimens in human skin fibroblasts and human osteosarcoma cells. The cell viability of PEO-pretreated Ta2O5 films even two-fold higher than the Ti specimens in human osteosarcoma cells. In addition, no cytotoxicity was detected in both the Ta2O5 and PEO-pretreated Ta2O5 films. It concluded that porous Ta2O5 film pretreated with PEO procedure possessed both great biocompatibility and antibacterial abilities. It might have highly potential to be a candidate for the surface materials of dental or orthopedic implants.

Effect of ion bombardment on the chemical properties of crystalline tantalum pentoxide films

The effect of argon ion bombardment on the chemical properties of crystalline Ta2O5 films grown on Si(100) substrates by radio frequency magnetron sputtering was investigated by X-ray photoelectron spectroscopy. All samples were irradiated for several time intervals [(0.5, 3, 6, 9) min] and the Ta 4f and O 1s core levels were measured each time. Upon analysis at the surface of the films, we observe the Ta 4f spectrum characteristic of Ta2O5. Irradiated samples exhibit the formation of Ta suboxides with oxidation states Ta1+, Ta2+, Ta3+ Ta4+, and Ta5+. Exposing the films, after ion bombardment, to ambient for some days stimulates the amorphous phase of Ta2O5 at the surface suggesting that the suboxides of Ta are unstable. Using a sputtering simulation we discuss that these suboxides are largely generated during ion bombardment that greatly reduces the oxygen to tantalum ratio as the irradiation time increases. The computer simulation indicates that this is due to the high sputtering yield of oxygen.

Improved pH Sensitivity and Reliability for Extended Gate Field-Effect Transistor Sensors Using High-k Sensing Membranes.

In this study, we fabricated extended-gate (EG) field-effect transistor (FET) pH sensors with dualgate (DG) structures, using a range of dielectric sensing membranes (SiO₂, Si₃N₄, HfO₂ and Ta₂O5) to vary their sensitivity. The fabricated EGFETs consisted of a silicon-on-insulator (SOI)-based metal-oxide semiconductor field-effect transistor (MOSFET) transducer and an EG sensor. We amplified the sensitivity of the device far beyond the Nernst limit (59 mV/pH), which is the theoretical maximum of conventional ion-selective FET (ISFET) sensing, by applying capacitive coupling. Among the evaluated dielectric sensing membranes, we obtained the highest sensitivity (478 mV/pH), low hysteresis (100.2 mV) and drift rate (24.6 mV/h) from the pH sensor with a Ta₂O5 membrane. Hence, we expect DG FET configurations using Ta₂O5 films as EG sensing membranes to be useful for high performance biosensor applications, as they satisfy the requirements for sensitivity, stability and reliability.

Investigation of Ta2O5 as an Alternative High-<inline-formula> <tex-math notation="LaTeX">${k}$ </tex-math> </inline-formula> Dielectric for InAlN/GaN MOS-HEMT on Si

We report on the demonstration and investigation of Ta2O5 as high-\k{appa} dielectric for InAlN/GaN-MOS HEMT-on-Si. Ta2O5 of thickness 24 nm and dielectric constant ~ 30 was sputter deposited on InAlN/GaN HEMT and was investigated for different post deposition anneal conditions (PDA). The gate leakage was 16nA/mm at -15 V which was ~ 5 orders of magnitude lower compared to reference HEMT. The 2-dimensional electron gas (2DEG) density was found to vary with annealing temperature suggesting the presence of net charge at the Ta2O5/InAlN interface. Dispersion in the capacitance-voltage (C-V) characteristics was used to estimate the frequency-dependent interface charge while energy band diagrams under flat band conditions were investigated to estimate fixed charge. The optimum anneal condition was found to be 500{\deg} C which has resulted into a flat band voltage spread (VFB) of 0.4 V and interface fix charge (Qf) of 3.98x10^13 cm-2. XPS (X-ray photoelectron spectroscopy) spectra of as deposited and annealed Ta2O5 film were analyzed for Ta and O compositions in the film. The sample annealed at 500{\deg} C has shown Ta:O ratio of 0.41.XRD (X-ray diffraction) analysis was done to check the evolution of poly-crystallization of the Ta2O5 film at higher annealing temperatures.

Effect of sodium addition and thermal annealing on second-order optical nonlinearity in thermally poled amorphous Ta2O5 thin films

We demonstrate second-order optical nonlinearity (SON) induced in amorphous tantalum pentoxide (Ta2O5) films by thermal poling. The effects of sodium addition and thermal annealing on the efficiency and stability of second harmonic generation (SHG) have been investigated. Maker fringe analysis has been used to quantify the poling-induced SON. χ(2) of 0.46 ± 0.02 pm/V and 0.34 ± 0.02 pm/V was achieved for sodium-containing and pure Ta2O5 films, respectively. Both the strength and the temporal stability of the induced nonlinearity are discussed with respect to the poling mechanisms and measured electrical conductivity. The lower density of uncombined charges of the Ta2O5 thin films is believed to play a role in the strength of SHG signal, and the rate of SHG decay decreases with thermal annealing.

Local structural investigation of refractory oxide thin films near laser damage threshold

Study of laser induced damage mechanism and effort to increase the damage threshold of oxide thin films is of immense technological importance because as improved laser-damage resistant optical materials and better fabrication technologies are developed, laser designers can increase the system operating energies and powers to the limits of these new materials. Although, many efforts have been made to investigate how stoichiometry or long range order structure plays a role in describing the laser damage process, to the best of our knowledge, efforts to understand the local structural changes in the materials due to laser irradiation around the damage threshold has not been made so far. In this communication, a set of RF sputter deposited dielectric refractory oxide thin film samples have been subjected to laser irradiation across their damage threshold. In case of TiO2, Ta2O5, and HfO2 thin films, the samples were irradiated with optimized laser fluence and different locations on the samples were irradiated with varying number of pulse shots, while in case of Gd2O3 films, different locations of the samples were irradiated with fixed number of pulse shots and with varying laser fluence. The irradiated samples have subsequently been subjected to X-ray absorption spectroscopy (XAS) studies to find out change in the local structure of the samples under laser irradiation. Apart from XAS measurements, various other characterization techniques such as optical microscopic imaging, grazing incidence X-ray reflectivity and grazing incidence X-ray diffraction have also been employed to study the laser irradiated zones. The results indicate interesting local structural evolution in oxide thin films with the onset of laser induced damage which gives novel insight about the laser damage mechanism.

Optical spectrometer based on continuously-chirped guided mode resonance filter.

In this work we introduce a tunable GMR filter based on continuously period-chirped (ΔP = 130 nm) gratings using a Ta2O5 waveguide layer with graded thickness (ΔT = 36 nm). The structure of the gradient-period grating is defined using a modified Lloyd's mirror interferometer with a convex mirror, and Ta2O5 film used for the gradient is deposited using masked e-beam evaporation. The as-realized chirped GMR filter provides sharp transmission dips at resonant wavelengths with a filter bandwidth of approximately 4.2 nm and 0.78 nm when respectively applied to TE and TM polarized light under normal incidence. Gradually sweeping the chirped GMR filter makes it possible to monotonically sweep through resonant wavelengths from 500 to 700 nm, while maintaining stable filter bandwidth and transmission intensity. The optical spectrum of the incoming light can then be loyally reconstructed accordingly. We successfully demonstrate the spectrum reconstruction of a white light emitting diode and a dual-peak laser beam using the proposed chirped GMR filter as a dispersive device.

Laser Induced Damage Threshold of Ta2O5 and Ta2O5/SiO2 Films at 532 and 1064 nm

Laser induced damage threshold (LIDT) of conventional electron beam deposited Ta2O5 and SiO2 thin films on BK-7 substrate are studied to see the effect of multilayer boundary and related effect for their combination as high and low index material films when number of layers increases as well as improvement on upper two non-quarter wave thickness layers. Reflection spectra of these samples show a relatively small decrease in reflection in non-quarter wave layer design and Grazing Incidence X-ray Diffraction (GIXRD) patterns show that deposited films are amorphous, having high (4.2 eV) band gap, sufficient high refractive index and very low extinction coefficient or low absorption which are factors favourable for high laser induced damage threshold. LIDT measured for single layer at 1064 nm is higher than that of multilayers and vice-versa at 532 nm wavelength. However, improvement of LIDT with non- quarter wave outer layers is more at 532 nm than 1064 nm.

A MODEL BASED METHOD FOR COMPARING THE PROPERTIES OF SOME METAL-Ta2O5/SiO2-Si STRUCTURES

Oxygen annealed radio frequency (RF) reactively sputtered and thermally grown (thermal) Ta2O5 films on sili-con were comparatively studied by using combination of C-V and I-V measurements and the previously developed comprehensive model for the metal-Ta2O5/SiO2-Si structures. Dielectric properties of separate layers were extracted by comparing the experimental and the theoretical results. It is found that the net leakage properties of the Ta2O5 layer are significantly better in the case of RF than thermal, particularly in the case of the Au gate. Excessive growth of the SiO2 layer of about 0.3 nm in the case of RF films leads to an unwanted increase of the equivalent oxide thickness. Appropriate interface engineering is required in order to prevent the SiO2 excessive growth during the oxygen anneal-ing. Such a growth can reduce the beneficial effects of the annealing on the net properties of Ta2O5 films obtained by RF.

Deposition and characterization of RF-sputtered-Ta2O5 thin films for O2 reduction reaction in polymer electrolyte membrane fuel cells (PEMFC)

Ta2O5 thin films, proposed as the replacement of the precious Pt-based electro-catalyst in fuel cells, were sputtered on laser textured silicon substrate at 200 °C and then air annealed at 350 °C, 400 °C and 450 °C with the aim to improve crystallinity and uniformity. Characterization such as FESEM, XRD, Hall effect and Cyclic Voltammetry were employed to investigate the morphological, structural, electrical and electrochemical properties of the as-sputtered as well as annealed samples. Initial results showed that the films obtained by increasing the substrate temperature during sputtering are smoother and have better adhesion to the etched silicon substrates. It has also been observed that the rate of deposition increases resulting in thicker films for longer deposition time at higher temperature. Upon annealing, Ta2O5 films achieved better crystallinity consisting of orthorhombic phases. The average thicknesses of the films are in the range of 400 nm–700 nm. The proposed catalyst also shows better enhancement for oxygen reduction reaction (ORR) in prolonged time of continuous potentiostatic electrolysis as to be used in fuel cells.

High capacity and performance lithium based electrochromic device via amorphous tantalum oxide protective layer

Electrochromic devices (ECDs) composed tungsten oxide (WOx) and nickel oxide (NiOx) films have attracted extensive attention due to the high transmittance contrast, the short response time and the low power consumption which represent a better prospect of applications. Even though a lot of efforts have been made to improve the stabilities and the cycling life of the ECDs, however, the effects are unsatisfactory, because the physical mechanisms of the degenerate properties are still not clear. Meanwhile, an easy processing and low cost manufacture method by composing functional devices are still insufficient. We hereby present an effective method to obtain a high capacity and performance lithium based ECDs by using of amorphous tantalum oxide (Ta2O5) protective layers, which can be used to improve the surface morphologies of NiOx films, effectively protect the films from the slightly acid environment of the electrolyte and reduce the leakage. Five-layer structures ECD1 (ITO/NiOx/Li+ based gel polymer electrolyte/WOx/ITO) and six-layer structures ECD2 (ITO/NiOx/Ta2O5/Li+ based gel polymer electrolyte/WOx/ITO) are prepared by special laminated glass method. Through the measurement of surface morphology and surface structure, as well as the electrochemical and spectral characterization, the charge capacity of NiOx with the optimized Ta2O5 film increases by 38% compared with the pure NiOx film. Also, the Ta2O5 film is helpful to enlarge the transmittance contrast, decrease the peak current of potential switch and electrochemical impedance. In the present of the protective layer, the ECD2 shows better bleached state (72%), higher coloration efficiency (66.2 cm2/C) and most importantly, better cyclic stability.

Tri-state resistive switching characteristics of MnO/Ta2O5 resistive random access memory device by a controllable reset process

Tri-state resistive switching characteristics of bilayer resistive random access memory devices based on manganese oxide (MnO)/tantalum oxide (Ta2O5) have been studied. The current–voltage (I–V) characteristics of the Ag/MnO/Ta2O5/Pt device show tri-state resistive switching (RS) behavior with a high resistance state (HRS), intermediate resistance state (IRS), and low resistance state (LRS), which are controlled by the reset process. The MnO/Ta2O5 film shows bipolar RS behavior through the formation and rupture of conducting filaments without the forming process. The device shows reproducible and stable RS both from the HRS to the LRS and from the IRS to the LRS. In order to elucidate the tri-state RS mechanism in the Ag/MnO/Ta2O5/Pt device, transmission electron microscope (TEM) images are measured in the LRS, IRS and HRS. White lines like dendrites are observed in the Ta2O5 film in both the LRS and the IRS. Poole–Frenkel conduction, space charge limited conduction, and Ohmic conduction are proposed as the dominant conduction mechanisms for the Ag/MnO/Ta2O5/Pt device based on the obtained I–V characteristics and TEM images.

Study on the composite dispersion model of optical constants of metal-oxide films in the range from ultraviolet to near infrared

Metal-oxide films as important high refractive index material are widely used in coating technology from ultraviolet to near infrared region. The optical constants (refractive index and extinction coefficient) and band gap properties of the films are essential for the optical multilayer design and applications. Based on the Tauc-Lorentz and Cody-Lorentz dispersion models, the composite model for the inverse calculation of optical constants from ultraviolet to near infrared region was studied. The method is proposed to link up the interband transition absorption and transparent region absorption more reasonably. Problems such as the characterization of weak absorption in transparent region and the step cutoff of the absorption near band gap, which were caused by using single physical dispersion model, were solved. Two kinds of films (HfO2 films and Ta2O5 films) were prepared by ion beam sputtering. Reflectance and transmittance spectra of the prepared films were used as the complex object for inversion calculation of the optical constants by the developed model. The performance of the fitness was compared with that of single physical dispersion model. Results show that the developed physical dispersion model of optical constants has a explicit physical meaning and versatility in the range from ultraviolet to near infrared region.

Implementation of Low‐Power Electronic Devices Using Solution‐Processed Tantalum Pentoxide Dielectric

AbstractThe development of solution‐processed field effect transistors (FETs) based on organic and hybrid materials over the past two decades has demonstrated the incredible potential in these technologies. However, solution processed FETs generally require impracticably high voltages to switch on and off, which precludes their application in low‐power devices and prevent their integration with standard logic circuitry. Here, a universal and environmentally benign solution‐processing method for the preparation of Ta2O5, HfO2 and ZrO2 amorphous dielectric thin films is demonstrated. High mobility CdS FETs are fabricated on such high‐κ dielectric substrates entirely via solution‐processing. The highest mobility, 2.97 cm2 V−1 s−1 is achieved in the device with Ta2O5 dielectric with a low threshold voltage of 1.00 V, which is higher than the mobility of the reference CdS FET with SiO2 dielectric with an order of magnitude decrease in threshold voltage as well. Because these FETs can be operated at less than 5 V, they may potentially be integrated with existing logic and display circuitry without significant signal amplification. This report demonstrates high‐mobility FETs using solution‐processed Ta2O5 dielectrics with drastically reduced power consumption; ≈95% reduction compared to that of the device with a conventional SiO2 gate dielectric.

Influence of Ta2O5 Interfacial Oxide Layer Thickness on Electronic Parameters of Al/Ta2O5/p-Si/Al Heterostructure

We describe the impact of Ta2O5 interfacial oxide layer thickness (ranging from 100-350 nm) on electrical and structural properties of Al/Ta2O5/p-Si/Al Metal-Insulator-Semiconductor (MIS) Schottky barrier diodes using RF magnetron sputtering. We studied the Schottky barrier device parameters such as ideality factor, barrier height and series resistance and are evaluated from current-voltage (I-V) measurements. The barrier height and ideality factor values are significantly varying with Ta2O5 oxide layer thickness and found to be 0.58 eV, 2.35, 0.71 eV, 2.10 and 0.78 eV, 1.87 for 20, 40 and 60 nm, respectively. It was noticed that the calculated barrier height and ideality values for this prepared Al/Ta2O5/p-Si/Al MIS Schottky barrier diode were greatly improved than those conventional metal-semiconductor (MS) Schottky diodes. The XRD studies revealed that the 100-nm thickness film exhibited poor crystallinity whereas 200 and 350 nm thickness films showed improved crystallinity with orthorhombic phase of β-Ta2O5. The presence of this orthorhombic phase of β-Ta2O5 is confirmed with FTIR studies. To explore the structural transformations in Ta2O5 films with varying thicknesses, Raman spectroscopy was utilized. In addition, the improvement in Schottky diode parameters was correlated with the enhanced crystallinity noticed in XRD studies.

Effects of Annealing on Residual Stress in Ta2O5 Films Deposited by Dual Ion Beam Sputtering

Optical coatings deposited by the dual ion beam sputtering (DIBS) method usually show high compressive stress, which results in severe wavefront deformation of optical elements. Annealing post-treatment has been widely used to control the residual stress of optical coatings. However, the effect of annealing on the stress of Ta2O5 films deposited by the IBS method has not been reported in detail. In this study, different thicknesses of Ta2O5 films were deposited by IBS and annealed at different temperatures from 473 to 973 K in air, and the effect of annealing on the stress of Ta2O5 films was investigated. The as-deposited Ta2O5 films deposited by IBS show high compressive stress, which are about 160 MPa. The compressive stress decreases linearly with the increasing temperature, and the wavefront deformation of Ta2O5 films increases linearly with film thickness (within 20 μm) at the same annealing temperature. When the temperature rises to 591 K, Ta2O5 films with zero-stress can be obtained. Ta2O5 films show tensile stress instead of compressive stress with further increasing annealing temperature, and the tensile stress increases with increasing temperature. Meanwhile, with the increasing annealing temperature, the refractive index of Ta2O5 film decreases, indicating the decreasing packing density. The atomic force microscope (AFM) test results show that surface roughness of Ta2O5 films slowly increases with the increasing of annealing temperature. Moreover, X-ray photoelectron spectroscopy (XPS) analysis shows that the Ta in Ta2O5 films can be further oxidized with increasing annealing temperature, namely, the absorption of Ta2O5 film can be reduced. X-ray diffraction (XRD) analysis shows that the annealing temperature should be below 923 K to maintain the amorphous structure of the Ta2O5 film.

Tantalum oxide film deposited by vacuum cathodic arc plasma with improved electrochromic performance

Inorganic solid-state electrochromic devices (ECDs) based on tantalum oxide (Ta2O5) as ion conductor layer were fabricated by vacuum cathodic arc plasma (CAP) deposition. This work focuses on key factor of fabricating Ta2O5 film deposited by CAP through four different ratios of oxygen and argon. Our results show that refractive index mostly decreased with increasing oxygen flow rate. Refractive index of 1.9 was achieved for Ta 2O5 film with O2/Ar = 2.4 which provided high ion conduction pathways through a highly porous structure. We also observed higher ionic conductivity σi=3.5×10−6Scm−1 for Ta2O5 film at O2/Ar = 2.4 which enhanced ion's mobility for ECDs causing rapid coloring/bleaching phenomenon. As a result, excellent optical density and rapid response times were observed in fabricated device with an area of 20 cm2, exhibiting optical density of 0.63 (@550 nm) and transmittance variation T = 50% (@550 nm) with the bleaching time= 5 s and transmittance variation T = 50% (@550 nm) with the coloring time = 13 s.

Effects of oxygen flows on optical properties, micro-structure and residual stress of Ta2O5 films deposited by DIBS

High precision optical components, such as ultra-high reflectors, severely suffer from absorption losses. In order to get optical films with low absorption losses, many techniques, such as annealing and laser conditioning, have been widely employed as a post-deposition treatment process to deal with as-deposited films. However, the success of these processes starts with the correct choice of the deposition parameters. In this paper, Ta2O5 films are deposited by dual ion beam sputtering (DIBS) method and the effects of target oxygen flows on optical properties, microstructure and residual stress are systematically investigated. The results show that target oxygen flows can significantly affect the absorption losses and residual stress of Ta2O5 film. The XRD patterns reveal that Ta2O5 films deposited at different parameters are all amorphous. Finally, based on the results of Ta2O5 films, broadband dielectric mirror with high reflection and low absorption losses in the range from 380 nm to 750 nm is successfully fabricated.

Medium band gap polymer based solution-processed high-κ composite gate dielectrics for ambipolar OFET

The authors present a novel ambipolar organic filed-effect transistors (OFETs) composed of a hybrid dielectric thin film of Ta2O5:PMMA nanocomposite material, and solution processed poly(selenophene, benzotriazole and dialkoxy substituted [1,2-b:4, 5-b′] dithiophene (P-SBTBDT)-based organic semiconducting material as the active layer of the device. We find that the Ta2O5:PMMA insulator shows n-type conduction character, and its combination with the p-type P-SBTBDT organic semiconductor leads to an ambipolar OFET device. Top-gated OFETs were fabricated on glass substrate consisting of interdigitated ITO electrodes. P-SBTBDT-based material was spin coated on the interdigitated ITO electrodes. Subsequently, a solution processed Ta2O5:PMMA nanocomposite material was spin coated, thereby creating the gate dielectric layer. Finally, as a gate metal, an aluminum layer was deposited by thermal evaporation. The fabricated OFETs exhibited an ambipolar performance with good air-stability, high field-induced current and relatively high electron and hole mobilities although Ta2O5:PMMA nanocomposite films have slightly higher leakage current compared to the pure Ta2O5 films. Dielectric properties of the devices with different ratios of Ta2O5:PMMA were also investigated. The dielectric constant varied between 3.6 and 5.3 at 100 Hz, depending on the Ta2O5:PMMA ratio.

The Role of Defects in the Resistive Switching Behavior of Ta2O5-TiO2-Based Metal–Insulator–Metal (MIM) Devices for Memory Applications

We describe the role of defects in the resistive switching behavior of metal–insulator–metal devices based on alternating Ta2O5 and TiO2 stacks. Ruthenium oxide (RuOx) and platinum (Pt) were used as bottom and top electrodes, respectively. Insulator stacks with thickness of 5 nm were fabricated by atomic layer deposition of alternating Ta2O5 and TiO2 thin films. Bipolar resistive switching behavior was obtained for Ta2O5-TiO2-Ta2O5 and TiO2-Ta2O5-TiO2 stacks, being mainly due to presence of oxygen vacancy defects. The best memristive response was obtained in the case of two TiO2 films embedding a monolayer of Ta2O5. Highly repeatable direct-current (DC)-voltage bipolar switching cycles were obtained. Small-signal admittance parameters also showed hysteretic behavior during a whole bipolar switching cycle. In the case of samples with three layers of similar thickness, when the transition from ON to OFF state (reset) occurred, the conductance abruptly increased and the susceptance decreased quickly for more negative voltages values. Such behavior was not observed when only one Ta2O5 monolayer was examined. These differences can be explained in terms of the charge transport mechanism occurring in the open conductive filaments.